According to JP2002-327576A, an apparatus automatically drives (opens and closes) a slide door for a side opening of an automobile. The apparatus comprises a housing, an output drum rotatably supported on the housing through a rotatable shaft, a motor for generating a driving torque and a clutch for selectively transmitting the driving torque from the motor to the output drum. A reduction mechanism is interposed between the output drum and the motor. A cable is installed between the output drum and an input drum connected to the slide door so that the driving torque is transmitted from the output drum to the slide door. The output drum, the reduction mechanism and the clutch are coaxially installed on the shaft. The apparatus is complicated in construction so that minitualization thereof is restricted.